A magnetic coil may carry a large DC current and an AC ripple current. Any AC loss in the magnetic coil, even when the AC current is small compared to the DC current, may be significant.
One method of reducing AC losses (sometimes described in terms of reducing AC resistance) in a magnetic coil is to use litzendraht (litz) wire. Litz wire is constructed from a plurality of insulated wire strands, and, in theory, has lower AC resistance than a single wire strand of the same cross-sectional area. An AC current travels near the surface of a conductor; an effect known as skin effect. Litz wire may reduce this skin effect when properly twisted and woven. Another effect that causes losses in a magnetic coil is the proximity effect, which occurs where the magnetic field created by a first wire or strand produces eddy currents in a second wire or strand. Litz wire may reduce proximity effect.
One disadvantage of litz wire is that it has a higher DC resistance, compared to a single strand wire of the same cross sectional area, making it undesirable for applications where DC current is large compared to AC current. Litz wire also has a higher cost than single strand wire and foil.
Another technique for reducing AC losses in a magnetic coil is to use an optimized-shape wire winding that positions wire (which may be litz wire) away from any gaps in a magnetic core. Disadvantages of using optimized shape wire winding include a more difficult and expensive winding and, if litz wire is used, the same increased DC resistance.
Yet another technique for reducing AC losses in a magnetic coil is to use multiple small gaps in the magnetic core instead of a single large gap. However, this increases the cost of the magnetic coil; it has also been shown that an optimized winding shape may be superior to the use of the multi-gapped magnetic core approach.
Typically, magnetic coils that carry high DC current (e.g., high power inductors, flyback transformers, etc.) are constructed with foil windings. Foil windings have low DC resistance, but, as with a multi-layer winding, AC losses are, in some cases, proportional to the square of the number of layers. Magnetic coils used in power applications typically require an air gap in the magnetic core to prevent magnetic saturation, to control inductance and to store magnetic energy. In high frequency applications (also in low-frequency applications), such as those incorporating switching power converters the magnetic field near this air gap induces large AC losses in the magnetic coil, particularly, in portions of the winding near the gap.
Although the above techniques, particularly the optimized shape winding, may be effective, the DC current in designs incorporating these techniques is much larger than the AC current; it is therefore not acceptable to significantly increase DC resistance. For high DC current windings, copper foil is often used since it is possible to achieve a higher packing factor (the portion of the winding window containing copper) than can be achieved with round wire. However, copper foil windings are particularly susceptible to induced eddy current from the gap fringing field. This is because the fringing field contains magnetic flux components perpendicular to the plane of the foil, which can produce significant losses even when the AC current is much smaller than the DC current in the winding.
FIG. 1 illustrates an exploded three dimensional representation of a foil wound magnetic coil 10. Magnetic coil 10 is shown with a magnetic core 12 and a foil wound coil 14. Foil wound coil 14 is shown removed from center leg 16 of magnetic core 12 for purposes of illustration (i.e., in normal operation, center leg 16 extends through center hole 15 of foil wound coil 14; see FIG. 2, FIG. 3A, FIG. 3B). Center leg 16 has a gap 18 to prevent magnetic saturation of magnetic coil 10 during operation. Magnetic core 12 has two winding windows 20 and 22. FIG. 2 illustrates a front elevation of magnetic coil 10, showing magnetic core 12 with foil coil 14 installed on center leg 16.
FIG. 3A illustrates a vertical cross-section A-A through magnetic coil 10 with foil wound coil 14 installed on center leg 16 of magnetic core 12. FIG. 3A illustrates magnetic core 12 and a copper foil winding 30 of foil wound coil 14. Copper foil winding 30 is shown filling winding windows 20 and 22 of magnetic core 12.
FIG. 3B shows an enlargement 40 of an area around gap 18 of cross-section A-A, FIG. 3A. Enlargement 40 shows inner copper foil winding 30 surrounding center leg 16 and a gap fringing field 32 that occurs around gap 18 during operation of magnetic coil 10. In particular, gap fringing field 32 induces eddy currents in copper foil winding 30 that increase AC losses, particularly for high frequency AC currents.
It is thus desirable to remove or minimize the effect of gap fringing field 32 on foil winding 30 to reduce AC losses without significantly increasing the DC resistance of magnetic coil 10.